T. Dohi and R. Kikinis (Eds.): MICCAI 2002, LNCS 2488, pp. 182–191, 2002.© Springer-Verlag Berlin Heidelberg 2002

Safety-Active Catheter with Multiple-Segments Drivenby Micro-hydraulic Actuators

Koji Ikuta, Hironobu Ichikawa, and Katsuya Suzuki

Nagoya University, Graduate School of Engineering,Department of Micro System Engineering,

Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8603, JapanTel: +81 52-789-5024, Fax: +81 52-789-5027

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Abstract. We proposed and developed an innovative active catheter with multi-segments that can bend in the narrow blood vessel. Micro hydraulic actuatorsystem based on new principle has been developed by the authors. Moreover,new micro fabrication method named hybrid stereolithograpy (IH process) re-quiring any assemble process is introduced for leakage-free packaging catheter.Total system with pressure control system was made. Good safety and driveperformance were verified experimentally. We also devised theoretical modelsof these valves to facilitate quantitative design and to extend applications forsafe medical tools.

1 Introduction

Non-invasive or minimally invasive examinations and medical treatments are in-creasing in number as society ages. Catheterization is a typical minimally invasivemedical technique. A catheter is a thin tube which is inserted into a peripheral vesselin an upper or lower limb for diagnosis or treatment of vascular system diseases; it ismainly used to measure internal pressure of the cardiovascular system, collect bloodsamples and inject contract medium for angiography. Catheterization is popular inclinical surgery because it allows minimally invasive diagnosis.

However, it is very difficult to insert a catheter with a guide wire which is nowwidely used, due to not only small inner diameter but also bending, twisting and fre-quent branching of vessels. Therefore, there is a need to develop an active catheterwhich can bend and twist its shaft in any direction and choose a direction at a branchpoint of a vessel [1][2][3]. These studies attached electric actuators on the parts to beinserted into the body, such as SMA or a special polymer that requires a high electriccurrent of several amperes; this could be dangerous, with a risk of electric leakage incase of an accident such as breakage. There are also other problems that prevent prac-tical application such as poor durability due to the complex internal mechanism of acatheter, and increasing lines associated with increasing joints [4].

To solve these problems, this study employed normal physiological salt solutionfor both drive and drive signal transmitter, and proposed a concept of a novel, safety-active catheter with simple mechanism using a hydraulic actuator. A prototype of anew type of microvalve, which is the key to this catheter, was developed and exam-ined. The valve was then miniaturized to fit into an actual catheter. The control sys-tem was developed and safe actuation in the model blood vessel was verified.

Safety-Active Catheter with Multiple-Segments Driven by Micro-hydraulic Actuators 183

2 Proposal of Hydraulic Active Catheter

Figure 1 shows the concept of the hydraulic active catheter proposed in this study.The features of this system are as follows:

1) Electricity is not used on any part to be inserted into the body.2) The drive fluid also works as a signal transmitter.3) The number of diving lines will not increase with the number of active joints

due to the single drive system.In summary, this system consists of bellows as an actuator at each joint and a drive

tube to control the bellows as shown in the figure.Each joint is designed to bend while a bellows, as an actuator, is extended by fluid

supply. Since normal saline is used as the drive fluid, safety is ensured because onlyharmless normal saline, instead of an electric leak, would leak if the catheter were tobreak inside the body.

The problem is how to achieve compact wiring in designing micromachinery. If asignal line is required for each new joint, reliability is impaired due to complicatedsignal lines at the proximal end, especially in the case of a long, thin system like acatheter. Our proposed single drive system allows a multi-joint design, free fromcomplicated signal lines.

3 Band Pass Valve (BPV)

In this study, we developed a so-called “Single-input, multi-output” control mecha-nism which uses drive fluid that can also act as a signal transmitter and which controlseach bellows (valve) independently through the drive fluid. We invented a static pres-sure Band Pass Valve (BPV) for independent active drive to ensure a simple mecha-nism and ease of control.

The concept of the BPV drive is shown in Fig. 2. This valve consists of a pair ofvalves: a High Pass Valve (HPV) and a Low Pass Valve (LPV). When the inner pres-sure of the tube (a drive system) reaches specific pressure P1, the lower valve (HPV)opens to make BPV open, and therefore drive fluid is supplied to the bellows (a driveactuator). When the inner pressure increases to P2, the upper valve (LPV) shuts tomake BPV shut, and therefore the mechanism stops supplying fluid to the bellows.

Through these actions, BPV remains open within the preset specified pressurerange only (from P1 to P2 of inner tube pressure in the figure) and BPV supplies drivefluid into the bellows.

Figure 3 shows the general scheme of the single-input, multi-output controlmechanism of multi-joint active catheter using this BPV. The application of threejoints is shown in this figure. A set of bellows (an actuator) and BPV corresponds to ajoint, and they are arranged parallel to the drive tube. The drive pressure range ofBPV on each joint is set to be different from each other.

By increasing the inner pressure of the drive tube stepwise to the activation pres-sure band of each valve, only the target valve opens and fluid is discharged each time.Therefore, only the target bellows connected to the valve is supplied with fluid andextended, and only the target joint will bend.

184 K. Ikuta, H. Ichikawa, and K. Suzuki

Fig. 1. Basic concept of Micro Hydraulic Fig. 2. Basic Principle of Band Pass ValveActive Catheter with Multi-degrees of Freedom

Fig. 3. The one input multi-output controlmechanism

In this way, any valve state can be controlled by regulating the inner pressure ofone drive tube using the Band Pass Valve (BPV), and so each associated joint can becontrolled.

4 Design and Development of a Prototype Band Pass Valve (BPV)

To configure the proposed BPV, it is necessary to develop a pair of valves as men-tioned above, HPV and LPV.

4.1 Design of HPV

Figure 4 shows the basic structure of the High Pass Valve (HPV). This valve isstructured to block the flow path with a core material which is pressed by an elasticmembrane.

No leakage occurs from HPV in its closed state due to the complete blocking of theflow path with the core material. When the inner pressure exceeds the elastic mem-brane pressure against the core material, the valve opens as the core material is lifted,and fluid is discharged.

4.2 Design of LPV

Figure 5 shows the basic structure of the Low Pass Valve (LPV). This valve is struc-tured such that its elastic membrane covers a keyhole shaped channel.

When the inner pressure is low, fluid discharges from the gap between the flowpath and the elastic membrane. When the inner pressure increases, the elastic mem-brane is pressed to block the flow path and thus fluid discharge is stopped.

Safety-Active Catheter with Multiple-Segments Driven by Micro-hydraulic Actuators 185

4.3 Development of a Prototype Band Pass Valve (BPV)

Prototypes of HPV and LPV were developed as to verify the mechanism based on thevalve designs described in sections 4.1 and 4.2. Then, by connecting these two proto-type valves, a prototype BPV was made as shown in Fig. 6.

Fig. 4. Basic structure of HPV Fig. 5. Basic structure of LPV

Fig. 6. Prototype of BPV Fig. 7. Experimental evaluation on Pressure dependent flow rate of BPV

4.4 Characteristic Evaluation of BPV

The characteristics of the prototype BPV were evaluated. Figure 7 shows the testresults of the relationship between the inner pressure and the outflow discharge of theprototype BPV. These data verified showed that in the band pass drive, fluid passedthrough the valve only when a specific pressure was applied.

5 Construction of Theoretical Model for Quantitative Design

Theoretical models of two component valves of BPV, the LPV and HPV, were con-structed in order to design the BPV quantitatively. In addition, an experiment to verifythe constructed model was conducted.

5.1 Theoretical Model of LPV

First, the theoretical model of LPV was developed as shown in Fig. 8. Bernoulli’stheorem was applied to this model to derive Eq. (1). Let k represent the modulus ofelasticity of a circular elastic membrane of 1 mm diameter. This equation states thatthe pressure difference to close the valve (Pclose) is inversely proportional to the cubeof the diameter d of the elastic membrane.

186 K. Ikuta, H. Ichikawa, and K. Suzuki

5.2 Theoretical Model of HPV

Next, the theoretical model of HPV was developed as shown in Fig. 9. Equation (2)was derived by considering the force balance on the valve. (F is a bias force applied tocore from elastic film.)

This equation states that the pressure difference to open the valve (Popen) is di-rectly proportional to the modulus of elasticity of the membrane.

(1) (2)

Fig. 8. Theoretical model Fig. 9. Theoretical modelof LPV of LPV

Fig. 10. The comparison Fig. 11. First prototype of anof experimental result and active catheter with four activetheoretical model segments for verification of basic principle

5.3 Verification Experiment of Basic Principle

An experiment was conducted using a prototype LPV to verify our theoretical model.Measurement data were collected on the relationship between the diameter of theelastic membrane which blocks the flow path and the pressure difference when thevalve is closed in this experiment. The results are shown in Fig. 10. The chart indi-cates that the data are very close to the ideal curve, and hence this theoretical formulaenables the LPV to be quantitatively designed to close at a given pressure.

However, the theoretical model of the HPV could not be verified because of diffi-culty of quantitatively measuring the modulus of elasticity of the membrane. Never-theless, the quantitative design with the developed model is sufficiently practicalowing to the very simple operation mechanism of the HPV.

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Safety-Active Catheter with Multiple-Segments Driven by Micro-hydraulic Actuators 187

6 Driving Experiment of the First Prototype

A prototype of a hydraulic active catheter was developed which has three joints withBPV of the prototype verification model. A driving experiment was conducted withthis catheter on a single-input, multi-output control mechanism (Fig. 11).

The results confirmed that each joint was driven independently by the pressurecontrol on one drive tube, verifying that a single-input, multi-output control mecha-nism using drive fluid as a signal transmitter has been achieved.

7 Miniaturization of Band Pass Valve

The driving experiments proved the usefulness of the Band Pass Valve (BPV). How-ever, the maximum diameter of the prototype active catheter used in the driving ex-periments was 6 mm, which is sufficient for experiments but too large for catheteruse, and therefore it should be made smaller. To do this, the size of the valve must bereduced.

7.1 Problems for Miniaturization

Generally, miniaturization involves the following problems:1) Assembly operation2) Bonding partIn this system, each part should be made smaller to reduce the size of the valve.

However, it is difficult to assemble such micro parts, and furthermore, the bondingpart for valves hinders miniaturization. Airtightness is another important issue as thefluid must not leak from the valve.

To solve these problems, we devised the Hybrid IH process [5] .

7.2 New Fabrication Method Using Hybrid Micro-stereolithography (IH Process)

The Hybrid IH process is a shaping method developed in our laboratory, by the proc-ess shown in Fig.12. In this shaping method, first, UV curable epoxy resin is cured bylaser and laminated as in the usual shaping method. Next, a part made from a differentmaterial is loaded in the middle of the shaping process, and a single piece is formed.

Fig. 12. Fabrication pro-cess of Hybrid microstereolithography (IHProcess)

The features of the Hybrid IH process are as follows:1) No need for assembly operation and bonding2) High airtightness3) 3D structures can be produced.

188 K. Ikuta, H. Ichikawa, and K. Suzuki

A leak-free miniature 3D structure can thus be easily produced by the Hybrid IHprocess.

7.3 Design of Micro-HPV and Micro-LPV

The following factors were taken into consideration to produce microvalves by theHybrid IH process.

1) Selection of desirable materials for Hybrid IH process2) Appropriate design for a single-piece styleConsidering these points, the LPV and HPV are designed to make the most of the

characteristics of the Hybrid IH process.The LPV in the verification model had a keyhole shaped flow path covered with

the circular elastic membrane. The shapes of the flow path and the elastic membranewere modified for miniaturization as shown in Fig. 13. The material of the elasticmembrane was replaced with silicone rubber and formed into a platy, legged shape.There is no need for a bonding surface because the part is held in an insertion struc-ture. The drive mechanism is the same as that in the verification model: when theinner pressure increases, the silicone rubber part changes shape, plugs the fluid pathand stops the fluid discharge.

As for the HPV, the core material and the elastic membrane blocked the flow pathin the verification model. Therefore it consisted of many parts and also the assemblywas complex. These parts were therefore integrated into one silicone rubber part inthe miniaturized model (Fig. 14); again, no bonding part is needed because the sili-cone rubber part is held in an insertion structure. The drive mechanism is also thesame as that in the verification model. No leakage occurs in the closed state due to thecomplete blocking of the flow path. When the inner pressure increases, fluid is dis-charged from a gap which is formed by distortion of the silicone rubber part.

Since the mounting location on the drive tube is taken into consideration in de-signing both valves, the whole system is sleek even after mounting the valves to thetube.

7.4 Fabrication of a Prototype Micro-BPV

Miniaturized prototypes of the LPV and HPV having the designs described abovewere combined to make the BPV. Figure 15 shows a comparison of a micro-BPVmade by the Hybrid IH process and a BPV unit used as a verification model in thedriving experiment.

The valve diameter is reduced from 6 mm in the verification model to 3 mm usingthe Hybrid IH process as the figure shows.

Fig. 13. Design of micro LPV Fig. 14. Design of micro HPV

Safety-Active Catheter with Multiple-Segments Driven by Micro-hydraulic Actuators 189

Fig. 15. Prototype of BPV and minia-turized BPV in 3mm diameter

Fig. 16. Cross section of an one segment ofactive catheter

8 Prototyping Two-Segments Active Catheter

A two-segments active catheter was constructed using the developed micro-BPV. Thebellows and BPV are arranged along the drive tube. The size of a joint is 40 mm inlength and 3 mm in diameter. The bellows as an actuator is made from silicone rub-ber; it extends easily on one side but not on the other, so supplied fluid readily bendsit.

Figure 17 shows an prototyped active catheter on the finger tip. Each segment canbe bent independently as shown in this figure. Master-slave pressure control systemshown in Figure 18 was made for simple control by the doctors. Pressure range isenough low so as to decrease risk of break.

Fig. 17. Miniaturized hydraulic active Fig. 18. Pressure control system for master-catheter with two segments slave bending of each segment

9 Insertion Experiment

An insertion experiment was performed using the constructed active catheter with twosegments

3mm

6mm

190 K. Ikuta, H. Ichikawa, and K. Suzuki

The pathway for this insertion experiment is 5 mm in width and 5 mm in depth.Figure 19 shows a running insertion experiment, confirming that the two-segmentsactive catheter is inserted smoothly. Moreover, it can easily be inserted into placeswhere one-joint catheters are difficult to insert. In another experiment in the branchedblood vessel, the active bending ability of this tool was verified as shown in Fig.20. Itwas much easy to select direction at the branching point.

Even a 3-mm diameter catheter can be used if the sites are confined. For future ap-plication to thin blood vessels such as capillaries, the system needs to be made evensmaller.

Fig. 19. Insertion experiment into blood Fig. 20. Selecting direction at in the branchedvessel model blood vessel model

10 Conclusion

An innovative hydraulic micro actuator system that controls multiple segments inde-pendently with single-input signal through fluid as a signal transmitter for safe medi-cal tools was developed. And the prototype hydraulic active catheter using thismechanism was developed successfully. A driving experiment was conducted withthe prototype active catheter and the results were favorable. Verification experimentsof the theoretical models of valves confirmed that quantitative design was made.Moreover, a valve unit having the same function was made smaller using the Hybridmicro streolithography (IH process), involving micromachining technology developedby the authors, and an actual-size two-segment active catheter was constructed withthese valves. The usefulness and safety feature of the developed active catheter wasdemonstrated by an insertion experiment.

References

1. Shuxiang Guo, Toshio Fukuda, Fumihito Arai, Makoto Negoro, Keisuke Oguro, A Studyon Active Catheter System Structure Experimental Results and Characteristic Evaluation ofActive Catheter with Multi DOF, Journal of the Robotics Society of Japan, Vol.14 No.6,pp.820-835, 1996.

Safety-Active Catheter with Multiple-Segments Driven by Micro-hydraulic Actuators 191

2. M.Tanimoto et al, Improvement of Safety in Telemedicime System, Proc. of 3rd RoboticsSymposia, pp.139-144,1998.

3. M.Esashi,Moption Control System by Micromachining, Plenary Proc. of 1998 JSME Con-ference on Robotics and Mechatronics (ROBOMEC 98), pp.5-13, 1998.

4. K.Ikuta, M.Nokata, Minimum Wire Drive of Multi Micro Actuators Journal of the Ro-botics Society of Japan, Vol.16, No.6, pp.791-797, 1998.

5. K. Ikuta, S. Maruo, T. Fujisawa and A. Yamada, "Micro Concentrator with Opto-sense Mi-cro Reactor for Biochemical IC Chip Family - 3D Composite Structure and ExperimentalVerification -," Proceedings of the IEEE International Workshop on Micro Electro Me-chanical Systems(MEMS99), 376-381, 1999.